[swift-evolution] [RFC] "Library Evolution Support in Swift ('Resilience')"

[Jordan Rose]

> On Feb 10, 2016, at 12:45, David Owens II <david at owensd.io> wrote:
> 
> 
>> On Feb 8, 2016, at 6:24 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> This last point is a specific case of a general tenet of Swift: the default behavior is safe. Where possible, choices made when an entity is first published should not limit its evolution in the future.
> 
> 
> Regarding this:
> 
>> Changing or removing a default value is permitted but discouraged; it may break or change the meaning of existing source code.
> 
> 
> Maybe I'm being dense, but how is something with a caveat of "discouraged" and "it may break or change" in-line with "the default behavior is safe"? I've got no qualms with putting the default value in the client code; I actually think that is fine.
> 
> However, my concern is that you will have different behavior depending on if you simply drop in the updated binary vs recompile against the binary. Even worse if you have multiple components within your app that link against the library. If only one of those components is recompiled on release, you now have a problem of conflicting behavior within your own app because of the client-side calling the API will be using different values.
> 
> It would seem that removing the default value could be permitted (though maybe still discouraged) because this will result in a compiler error in the scenario above. It's still possible to have different behavior in your components, but now it's no longer implicitly happening. However, changing the default parameter seems highly problematic.

This is a good point, and I think the "checker" tool described at the end of the document should warn about these kinds of changes as well. I was trying to distinguish between "changing this affects the ABI of your library and therefore breaks memory and type safety" and "changing this merely affects the behavior of your library but will not break memory and type safety", but maybe that's not such an important distinction.

It's important to note that "it may break or change the meaning of existing source code" is something that applies to any behavior change you make in a library; if version 2.0 of an opaque function 'foo' accesses global memory where it didn't before, existing clients may run into concurrency issues if they assumed the function was concurrency-safe. The specific twist for inlineable code (including default argument expressions) is that the change is triggered by recompiling the client, which is why any changes to inlineable code (including default argument expressions) should really preserve the existing contract of the API.

I actually considered leaving out the section about changing a default parameter, but that's no different from removing a default parameter and then immediately doing a second release with a new parameter added back. Calling it out explicitly is intended to serve as a warning more than an endorsement.

Jordan

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